The Yixian and Jiufotang fossil beds of northeastern China have yielded such treasures as the first beaked bird skeleton and impressions of soft tissues, hair and feathers from prehistoric mammals and reptiles. Now, evidence suggests that pyroclastic flows — high-speed rivers of rock and gas —buried these ancient creatures and left them well preserved for 120 million years.

How the early Cretaceous creatures settled on ancient lake bottoms and became mixed with volcanic debris has been a myste...